The present invention relates to a magnetic resonance imaging method for the acquisition of a three-dimensional dataset, where spatial encoding by three mutually orthogonal magnetic field gradients is performed such, that signal is readout under a read-gradient in one spatial direction k1 and spatial encoding in the other two spatial directions k2 and k3 is performed by applying phase encoding gradients in the other two spatial directions prior to signal acquisition and data acquisition is performed in a sequential manner such that at each acquisition step signal is acquired under the said readout gradient, but with different combinations of the two phase encoding gradients.
A method of this type is known as 3DFT (or volume) imaging described in Reference [0] M. Bomans, K. Hohne, G. Laub, A. Pommert, U. Tiede, Improvement of 3D acquisition and visualization in MRI. Magn. Reson. Imaging 9, 597-609 (1991).
The present invention relates generally to magnetic resonance imaging (=MRI) technology. It specifically relates to data acquisition and image reconstruction methods as well as to spatial encoding for MRI.
Magnetic resonance imaging is a relative new technology compared with computed tomography (=CT) and the first MR Image was published in 1973 by P. C. Lauterbur in “Image Formation by Induced Local Interactions: Examples of Employing Nuclear Magnetic Resonance”, Nature 242, 190491. It is primarily a medical imaging technique which most commonly used in radiology to visualize the structure and function of the body. It could provide detailed Images of the body in any plane. MRI provides much greater contrast between the different soft tissues of the body than CT does, making it especially useful in neurological, cardiovascular, and oncological imaging. It uses a powerful magnetic field to align the nuclear magnetization of hydrogen atoms in water in the body. Radio frequency fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body.
An MRI system typically establishes a homogenous magnetic field, generally along a central axis of a subject undergoing an MRI procedure. This homogenous main magnetic field affects the magnetic properties of the subject to be imaged by aligning the nuclear spins, in atoms and molecules forming the body tissue. If the orientation of the nuclear spins is perturbed out of alignment, the nuclei attempt to realign their spins with the field. Perturbation of the orientation of the nuclear spins is typically caused by application of radio frequency (RF) pulses tuned to the Larmor frequency of the material of interest. During the realignment process, the nuclei process about the direction of the main magnetic field and emit electromagnetic signals that may be detected by one or more RF detector coils placed on or around the subject.
Magnetic resonance imaging employs temporally and spatially variable magnetic fields to encode position by affecting the local Larmor frequency of spins. Gradient coils typically used for that purpose generate spatial encoding magnetic fields (=SEMs) which are superimposed on the main magnetic field. This allows to choose the localization of the image slices and also to provide phase encoding and frequency encoding. This encoding permits identification of the origin of resonance signals during image reconstruction. The image quality and resolution depends significantly on the strength and how the applied encoding fields can be controlled. Control of the gradient coils is generally performed in accordance with pre-established protocols or sequences at events, called pulse sequences, permitting different types of contrast mechanisms to be imaged.
In half Fourier imaging, part of the k-space data are left unacquired in order to shorten the measurement time in MRI, and the missing data are synthesized by exploiting the conjugate symmetry of k-space during reconstruction (also known as partial Fourier imaging). A rectangular sampling pattern is often used in multi-dimensional half Fourier imaging (see References [1], [2]).
Off-resonance effects and technical imperfections may lead to spatial phase variation in the acquired MR image, which destroys the conjugate symmetry of k-space. Two types of phase correction methods were developed to synthesize the unacquired data (see References [3], [4]). In these methods, the phase image in an intermediate result is completely replaced by an estimate during the data synthesis.
Recently developed k-space random sampling techniques (also known as “Compressed Sensing”, see Reference [5]) were combined with Half Fourier acquisition to accelerate MRI (see References [2], [6]). In Reference [2], missing data in half Fourier imaging was reconstructed at a separate step using Homodyne detection method proposed in Reference [3]. In Reference [6], missing data in half Fourier imaging was reconstructed at a separate step using iterative POCS method proposed in Reference [4]. In all above methods, reconstructed image phase was directly replaced by the estimated phase, which may be inaccurate in some regions with rapid phase variation. Significant reconstruction errors exist in regions with poor phase estimation in these methods with ‘direct’ phase replacement.
The present invention presents a way to substantially overcome one or more disadvantages of the above discussed existing methods.
One object of the present invention is to propose a data acquisition method for half Fourier imaging in 3D MRI, where data are acquired under a readout gradient for spatial encoding in the first spatial dimension (referred to as k1). Data in the 2nd and 3rd spatial dimension are spatially encoded according using two mutually orthogonal phase encoding gradients (referred to as k2 and k3). K-space undersampling is performed in the k2-k3 plane.
The k-space in said k2-k3 plane comprises five parts:                Part 1: symmetrically acquired area in central k-space.        Part 2: symmetrically acquired area, with higher spatial frequencies than that in Part 1.        Part 3: acquired area in lower half k-space, with higher spatial frequencies than that in Part 2.        Part 4: acquired area in upper half k-space, with higher spatial frequencies that that in Part 2.        Part 5: unacquired area in upper half k-space, with higher spatial frequencies than that in Part 4.        
Part 1 in said k-space is fully sampled.
Part 2 in said k-space is undersampled with uniform sampling density.
Part 3 in said k-space is undersampled, with lower uniform sampling density than that in Part 2.
Part 4 in said k-space is undersampled, with lower uniform sampling density than that in Part 3.
Part 5 in said k-space is completely unacquired.
One option of the shape of said Part 2 and Part 4 is elliptical.
One option of the shape of said Part 2 and Part 4 is rectangular.
One option of data acquisition is to apply regular undersampling in said Part 2, Part 3 and Part 4 (see Reference [7]).
One option to data acquisition is to apply random undersampling in said Part 2, Part 3 and Part 4.
Another object of the present invention is to propose an image reconstruction method for the data set acquired by said k-space sampling pattern.